Binding of La3+ to calmodulin and its effects on the interaction between calmodulin and calmodulin binding peptide, polistes mastoparan

Binding of La3+ to calmodulin (CaM) and its effects on the complexes of CaM and CaM-binding peptide, polistes mastoparan (Mas), were investigated by nuclear magnetic resonance (NMR) spectroscopy, fluorescence and circular dichroism spectroscopy, and by the fluorescence stopped-flow method. The four binding sites of La3+ on CaM were identified as the same as the binding sites of Ca2+ on CaM through NMR titration of La3+ to uniformly N-15-labeled CaM. La3+ showed a slightly higher affinity to the binding sites on the N-terminal domain of CaM than that to the C-terminal. Large differences between the H-1-N-15 heteronuclear single quantum coherence (HSQC) spectra of Ca4CaM and La4CaM suggest conformational differences between the two complexes. Fluorescence and CD spectra also exhibited structural differences. In the presence of Ca2+ and La3+, a hybrid complex, Ca2La2CaM, was formed, and the binding of La3+ to the N-terminal domain of CaM seemed preferable over binding to the C-terminal domain. Through fluorescence titration, it was shown that La4CaM and Ca2La2CaM had similar affinities to Mas as Ca4CaM. Fluorescence stopped-flow experiments showed that the dissociation rate of La3+ from the C-terminal domain of CaM was higher than that from the N-terminal. However, in the presence of Mas, the dissociation rate of La3+ decreased and the dissociation processes from both global domains were indistinguishable. In addition, compared with the case of Ca4CaM-Mas, the slower dissociations of Mas from La4CaM-Mas and Ca2La2CaM-Mas complexes indicate that in the presence of La3+, the CaM-Mas complex became kinetically inert. A possible role of La3+ in the Ca2+-CaM-dependent pathway is discussed.